Most of the printing in lithography is done in the two fluid offset mode. A lithographic master is generally mounted on a rigid master cylinder. The image areas of the master are ink receptive while the non-image areas are water receptive. First a fountain solution is applied to the plate to wet the background and then an oil base ink is used to ink the image area. A large number of rollers are used in the inking train for proper distribution and control of ink. Because of the presence of water in the background, the oil base ink is repelled from the non-image areas. A proper balance of water and ink is necessary to attain this response. The inked master comes in contact with the blanket cylinder where the image is transferred to a relatively soft elastomeric blanket. The paper passes between the blanket cylinder and the impression cylinder and the image is offset from the blanket to the paper. There are advantages in having the intermediate blanket for image transfer. First it is responsible for the excellent image quality, second it helps in increasing the plate life and third it permits the use of a wide variety of printing stock. Good image quality is obtained even with rough paper because of the resiliency of the blanket.
These advantages are lost when prints are directly made from the planographic master to the paper (direct printing). The degradation of halftone scales and poor solid area image fill-in are the immediate consequences.
Even in offset lithographic printing it is known that image quality strongly depends on paper quality. Most of the high quality printing is done on smooth, coated papers. This dependence becomes more significant in the direct printing mode. A close examination of the paper surface shows that the surface is very rough. The peak to peak variation in surface profile could be as large as 20 .mu. depending on the type of paper examined. In order to have true reproduction of the image from the master to the paper it is necessary to have complete contact between the two surfaces. Either the image surface or the paper surface has to deform to assure conformity between the surfaces. This kind of deformation cannot be expected from the paper itself, although there might be a very small contribution through bending over large distances. In offset lithography the necessary deformation is provided by the blanket. Conventional lithographic masters are relatively hard and when they are used for direct mode printing only the high spots on the paper surface receive ink giving a very mottled effect in the image area. In the offset mode each of the halftone dots is reasonably uniform while in the direct mode the dots are very nonuniform. This non-uniformity of the printed dot results in the degradation of halftones. For solid area prints direct mode printing reduces the ink coverage by leaving open white spots in the image area. One can see under a microscope that these small dots correspond to valleys in the paper surface. Some of these voids are as large as 4 to 5 mils in diameter making them visible at a normal viewing distance.
It is clear from this discussion that a high quality direct printing master must have conformability to the paper surface variation and this invention is directed towards defining structural requirements for such a master.